Alloy deterring shunt for conical tungsten evaporation sources



Nov. 3, 1970 v H. L. GRANT 3,538,305

ALLOY DETERRING SHUNT FOR CONICAL TUNGSTEN EVAPORATION SOURCES Filed May 16, 1969 FIG. I

I PRIOR ART 1 7 2o POWER SUPPLY w POWER SUPPLY INVENTOR HOWARD L. GRANT ATTORNEY United States Patent 3,538,305 ALLOY DETERRING SHUNT FOR CONICAL TUNGSTEN EVAPORATION SOURCES Howard L. Grant, Forest Heights, Md., assignor t0 the United States of America as represented by the Secretary of the Navy Filed May 16, 1969, Ser. No. 825,373 Int. Cl. F22b 1/30 US. Cl. 219-472 4 Claims ABSTRACT OF THE DISCLOSURE An evaporation source heating element for use in a vacuum deposition chamber including a conical tungsten wire coil adapted to hold the material to be deposited, and a tungsten Wire current shunt path for reducing the current flowing through the tapered end of the conical basket thereby preventing alloy formation.

STATEMENT OF GOVERNMENT INTEREST The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes Without the payment of any royalties thereon or therefor.

BACKGROUND OF THE INVENTION The present invention relates generally to heating elements for use in vacuum deposition chambers and, more particularly, to a conical heating element having a current shunt path for reducing localized hot spots on the surface of the heating element to deter the formation of alloys.

For many years now, electrical components as well as totally integrated electrical circuits have been manufactured in high vacuum deposition chambers. The vapor deposition process is well known in the art and need not be explained in detail herein; however, for purposes of completeness and clarity it can be described generally as follows. A substrate material having suitable mechanical and electrical properties is placed inside a high vacuum chamber. Adjacent to the substrate surface, and also within the vacuum chamber, is a heating element used to bring the material to be deposited onto the substrate up to its evaporating point. An external power source is coupled to the heating element to provide the required electrical energization. Many varied heating elements have been developed to provide various types of deposition, some of which produce self-evaporation and therefore are self-destructive and others of which serve merely to support and heat various materials placed therein for evaporation-One device which falls into the last mentioned category is a conical tungsten basket.

One of the major problems encountered in the vacuum evaporation of materials such as germanium at high evaporation rates from conical tungsten baskets has been the formation of an alloy at the apex of the basket. This alloy formation has been found to be directly attributable to the existence of a localized hot spot at the apex of the basket. Due to the high temperature existing at this point, the evaporation material gradually forms alloys with the tungsten wire which forms the conical basket resulting in a gradual reduction of the wire diameter with concomitant increase in temperature and further alloying. This process continues and ultimately results in an open heater circuit.

Scientists and engineers utilizing conical heating elements have found that good thin films of various materials could be obtained at low evaporation rates from tungsten baskets without the formation of alloys; however, at-

Patented Nov. 3, 1970 tempts to increase evaporation rates have resulted in deleterious changes in film properties which are directly attributable to the alloy contamination from the tungsten basket. Although many various approaches have been taken to solve this problem, none have heretofore produced desired results.

OBJECTS OF THE INVENTION An additional object of the present invention is to provide a conical heating'element capable of operating at high evaporation rates without introducing contaminating alloys onto the substrate.

SUMMARY OF THE INVENTION The present invention is summarized in that a heating element for evaporating a semiconductive material in a vacuum chamber includes a wire coil tapered at one end to form a conical basket for holding the material, the wire having two electrical terminals each of which extend from a respective end of the wire coil. A power source is coupled to the electrical terminals for establishing a flow of current through the wire coil to heat the material to form a molten globule which is contained within the wire coil. The heating element further includes a current shunt device coupled to the wire coil for reducing the current flow through the tapered end of the coil thereby reducing the heat generated at that tapered end to prevent the material and the wire from forming alloys.

An advantage of this invention is the provision of an economical heating element which is constructed to provide uniform temperatures throughout its length to thereby inhibit alloy impurity formation.

Other objects, advantages and novel features of the invention will become more fully apparent from the following detailed description of the preferred embodiment of the invention when considered in conjunction with the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 shows a typical prior art heating element; and FIG. 2 illustrates the preferred embodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT FIG. 1 shows a typical prior art evaporation source heating element 10 which is wound to form a conical basket having a Wide mouth 12 at one end thereof and an apex 14 at the other end. The heating element has two end terminals 16 which are coupled to a power source 18 via a switch 20. The heating element 10 is located within a high vacuum deposition chamber shown diagrammatically as dashed line 22.

It is noted at this point that the conical heating element 10 can be constructed from any desired material, such as tungsten or the like. It should be pointed out, however, that like tungsten the material selected for the heating element should have sufficient mechanical rigidity to be selfsupporting while additionally having the ability to withstand extremely high temperatures without breaking down.

As illustrated in FIG. 1, a material 30 such as germanium or the like can be placed within the conical tungsten basket 10 where it will be held for heating. When switch 20 is closed, thereby completing the electrical circuit between power source 18 and the heating element 10, the wire basket will heat the enclosed material causing it to evaporate. If the current flowing through the heating element is limited to the point where a low rate of evaporation is maintained, localized hot spots will not be formed and the purity of the resulting deposited film will be kept high. However, due to the close proximity of adjacent coils at the tapered end 14 of the heating element, a localized high temperature area is generated at the apex of the basket when high evaporation rates are attempted. This results in the formation of alloys of the material enclosed within the basket and the wire material used to form the basket which consequently introduces undesired impurities into the vacuum chamber. Referring now to FIG. 2, which illustrates the preferred embodiment of the present invention, similar numerals are used throughout to refer to similar components as shown in FIG. 1. The wire basket shown in FIG. 2 is substantially identical to that shown in FIG. 1; however, it includes a current shunt electrode which directly aifects the temperature of that part of the element in the vicinity of the apex 14. The preferred embodiment of the current shunt, as shown in FIG. 2, includes a length of wire 26, which is constructed of the same material used to form the heating element 10, split lengthwise along a diameter. The wire is welded at point 28 to the terminal 16 of the heating element which directly extends from apex point 14. The other end of the shunt wire extends into the interior of the conical basket adjacent the tapered end of the heating element without touching any other part of the wire basket. When current is applied to the heating basket, the material held therein will pass to a liquid state whereupon it will completely surround the extended end of the shunt to thereby establish a parallel current path. The additional parallel current path causes the current flowing in the lower coils of the heating element, as illustrated, to be reduced. Therefore, the temperature of the basket in the vicinity of the apex will be prevented from rising to that temperature which causes alloy formation. In this manner, localized high temperature areas along the length of the wire used to form the conical heating basket can be eliminated thereby deterring alloy formation and enabling the utilization of high evaporation rates. It can therefore be seen that the present invention pro vides a simple, economical, and reliable heating element for use in vacuum deposition which retards alloy formation and enables high purity film deposition.

It should be understood, of course, that the foregoing disclosure relates to only a preferred embodiment of the invention and that numerous modifications or alterations may be made thereto in light of the above teachings.

What is claimed and desired to be secured by Letters Patent of the United States is:

1. A heating element for evaporating a material in a vacuum chamber, comprising:

a wire coil tapered at one end to form a conical basket for holding said material, said wire having two electrical terminals, each extending from a respective end of said wire coil;

a power source coupled to said electrical terminals for establishing a flow of current through said wire coil thereby heating said material to form a molten globule contained within said wire coil; and

current shunt means coupled to said wire coil for reducing the current flow through the tapered end of said wire coil thereby reducing the heat generated at said tapered end to prevent said material from forming alloys.

2. The invention as recited in claim 1, wherein said current shunt means comprises:

a tungsten wire shunt, having one end connected to that one of said two electrical terminals extending from the tapered end of said wire coil, and having its opposite end extending into the interior of said conical basket adjacent the tapered end of said wire coil whereby it is surrounded by said molten globule of said material to thereby provide a shunt current path around said tapered end of said wire coil.

3. The invention as recited in claim 2, wherein:

said material is a semiconductor; and

said wire coil is formed of tungsten wire.

4. The invention as recited in claim 3, wherein said tungsten wire shunt has a cross sectional area which is approximately equal to half the cross sectional area of the tungsten wire used to form said wire coil.

References Cited UNITED STATES PATENTS 652,638 6/1900 Potter 338218 X 2,428,093 9/1947 Raymond 338-218 2,662,159 12/1953 Bilofsky et a1 219272 2,686,865 8/1954 Kelly 219-426 X 2,866,065 12/1958 .Hirsh 219-272 X 3,017,851 1/1962 Krause 11849 3,244,857 4/1966 Bertelsen et al 219-275 3,486,001 12/1969 Czarnowski 219-271 VOLODYMYR Y. MAYEWSKY, Primary Examiner US. Cl. X.R. 

